It is known to use X-rays for transmission imaging in baggage scanning facilities at airports and the like.
EP 0621959 describes a method and apparatus for X-ray inspection using high energy X-rays which permit discrimination on the basis of atomic number between materials exposed to the X-rays.
It also describes a method and apparatus by which the contents of objects such as steel shipping containers as used for road, rail and maritime freight, can be X-rayed and a mean atomic number profile generated of the contents for the analysis using conventional image analysis techniques. These methods and apparatus have been used to identify the presence of particular substances or combinations of substances within a container, whereby an alarm signal is generated if one or more criteria is satisfied so as for example to prevent the loading or subsequent transit of a container so identified.
In a preferred method as aforesaid for detecting the mean atomic number of a mass of material in an object (typically within a container and therefore hidden from view), comprises the steps of:    1. subjecting the material to high energy X-rays and determining the mean number NA of X-rays transmitted through the region thereof,    2. subjecting the same region of the material to X-rays having a significantly higher energy than the first mentioned X-rays and determining the mean number NB of the higher energy X-rays transmitted therethrough,    3. computing the value of the ratio NA to NB, and    4. determining from a look-up table and delivering as an output the average atomic number corresponding to the computed value of the NA/NB ratio.
By significantly higher is meant at least twice and typically five or six times or more the energy of the first mentioned high energy X-rays. Thus if the lower high energy X-rays are of the order of 1 MeV, the higher energy X-rays will be typically of the order of 5 or 6 MeV.
A preferred embodiment of the invention described in EP 0621959 comprises a single broad energy band X-ray source which projects a range of high energy X-rays of 1 MeV and above, towards the object; a composite detector which is placed beyond the object, and which on bombardment by transmitted X-rays produces substantially simultaneously:    a. a first component predominantly attributable to the higher energy component of the incident X-rays and    b. a second component predominantly attributable to the lower energy component of the incident X-rays;circuit means adapted to determine the light generated in scintillating crystals by the said two components; means for generating therefrom numerical values relating thereto; means for forming a ratio of one numerical value relative to the other; and a look-up table of atomic numbers and ratio values, the mean atomic number for material through which the X-rays have passed can be derived using the derived value of the said ratio, for the material in question.